In U.S. Pat. No. 3,687,514 it is shown that the mode dispersion in a multimode waveguide can be reduced by deliberately enhancing the mode conversion processes in the waveguide. This is accomplished by introducing "imperfections" in the waveguide, such as changes in the cross-sectional dimensions of the guide and/or changes in the orientation of the guide axis. Arrangements for fabricating optical fibers having such variations are described in U.S. Pat. Nos. 4,038,062; 3,666,348; 3,912,478; and the previously cited U.S. Pat. No. 3,687,514. However, the periodicity of the resulting spatial variations produced by all of these techniques is much greater than one would like. As a result, the net coupling length for the fiber is relatively large and the reduction in the dispersion is correspondingly small.
In an alternative technique, described in U.S. Pat. No. 3,909,110, refractive index variations are built into the preform from which the fiber is to be pulled. This, however, requires considerable care in the preparation of the preform and, hence, tends to be a relatively expensive method.